1. Field of the Invention
The present invention relates to conductors for high-voltage windings, preferably for high-voltage windings of a stator in a high-voltage generator, which conductors are stranded with two or more layers of strands electrically insulated from each other, around a central core, which core may be one of the strands of the conductor, air or other material.
The invention also relates to rotating electric machine of the type described herein.
The rotating electrical machines referred to in this context are e.g. synchronous machines, but also double-fed machines, applications in asynchronous static current converter cascades, outerpole machines and synchronous flux machines as well as alternating current machines intended primarily as generators in a power station for generating electric power.
Magnetic circuits referred to in this context, include a magnetic core of laminated, non-oriented or oriented, sheet or other material, for example amorphous or powder-based, or any other arrangement for the purpose of allowing an alternating magnetic flux, a winding, a cooling system, etc., and which may be arranged in the stator of the machine, in the rotor or in both.
2. Discussion of the Background
The stator winding in known generators traditionally consists of an electric conductor having a number of insulated rectangular wires, also known as strands, of copper, aluminium or other suitable metal. These strands are transposed (i.e. change place with each other) and are surrounded by a common insulation in such a way that the bundle of conductors acquires a rectangular cross section. The copper conductors are rectangular in order to reduce the eddy-current losses accomplished by keeping (the linear dimension in the direction of the magnetic field small). However, a conductor with rectangular cross section in a high-voltage insulation (i.e. in a conductor) has the disadvantage in that it provides a much greater field strength at the corners of the conductor, the corners thus becoming dimensioning for the thickness of the insulation. Therefore, from the point of view of optimal insulation thickness, circular conductors would be preferable.
Circular conductors can be constructed in a great number of different ways. The conductor may, for instance, consist of the following:
1) a solid rod of copper or other metal with circular cross section,
2) a conductor stranded from circular wires having the same or different diameters,
3) a conductor stranded from sectioned wires or,
4) a conductor compressed from a number of segments, each of which is in turn stranded from circular wires and then formed into a segment.
In order to reduce the conductor dimension it is sometimes compressed or compacted after stranding, thereby altering the shape of the strands in the outermost layer or in the whole conductor, which may costitute a disadvantage.
To ensure high energy transfer in voltage-transmission lines with a conductor for a given voltage, the current must be increased, which is only possible if the conductor area is increased. When the current becomes large this entails the drawback that the current distribution in the conductor becomes uneven (the current endeavours to reach the outer surface of the conductor) and what is known as a xe2x80x9cskin effectxe2x80x9d, current pinch effect, is obtained. To counteract this effect, conductors , according to prior art, having a large cross section ( greater than 1200 mm2 Cu) are produced, usually called Millikan conductors, i.e. conductors built up of a number of concentrically arranged wires which have subsequently been compressed and shaped. Such a conductor is often composed of 5 or 7 segments which are in turn insulated from each other. Such a construction is effective in reducing the current pinch effect in transmission and distribution cables for high-voltage.
It is previously known that, in distribution systems for high-voltage power transmission, all the strands in the cable have been insulated with varnish, for instance, in order to reduce the current pinch effect, see the publication Hitachi Cable Review, No. 11, August 1992, pages 3-6: xe2x80x9cAn EHV Bulk Power Transmission Line Made with Low Loss XLPE Cablexe2x80x9d. This publication also describes how a few of the strands in the outermost layer are left uninsulated in order to prevent differences in potential between the wire strands and the inner semi-conductor layer. No application of this technology on generator windings, however, is described.
With generators having conventionally designed stator windings as described above, the upper limit for generated voltage has been deemed to be 30 kV. This usually means that a generator must be connected to the power network via a transformer which steps up the voltage to the level of the power network, xe2x80x94in the range of 130-400 kV.
During the last decades, there have been increasing demands for rotating electric machines for higher voltages than what has previously been possible to design. The maximum voltage level which, according to the state of the art, has been possible to achieve for synchronous machines with a good yield in the coil production is around 25-30 kV. It is also commonly known that the connection of a synchronous machine/generator to a power network must take place via a xcex94/Y-connected so-called step-up transformer, since the voltage of the power network normally lies at a higher level than the voltage of the rotating electric machine. Thus, this transformer, and the synchronous machine, constitute integral parts of an installation. The transformer constitutes an extra cost and also has the disadvantage that the total efficiency of the system is reduced. If it were possible to manufacture machines for considerably higher voltages, the step-up transformer could thus be omitted.
Attempts to develop the generator for higher voltages have, however, been in progress for a long time. This is clear, for instance from xe2x80x9cElectrical Worldxe2x80x9d, Oct. 15, 1932, pages 524-525, which describes how a generator designed by Parson in 1929 was arranged for 33 kV. It also describes a generator in Langerbrugge, Belgium, which produced a voltage of 36 kV. Although the article also speculates on the possibility of increasing voltage levels still further, the development was curtailed by the concepts upon which these generators were based. This was primarily because of the shortcomings of the insulation system where varnish-impregnated layers of mica oil and paper were used in several separate layers.
Certain attempts to find a new approach as regards the design of synchronous machines are described, inter alia, in an article entitled xe2x80x9cWater-and-oil-cooled Turbogenerator TVM-300xe2x80x9d in J. Elektrotechnika, No. 1, 1970, pp. 6-8, in U.S. Pat. No. 4,429,244 xe2x80x9cStator of Generatorxe2x80x9d and in Russian patent document CCCP Patent 955369.
The water- and oil-cooled synchronous machine described in J. Elektrotechnika is intended for voltages up to 20 kV. The article describes a new insulation system consisting of oil/paper insulation, which makes it possible to immerse the stator completely in oil. The oil can then be used as a coolant while at the same time using it as insulation. To prevent oil in the stator from leaking out towards the rotor, a dielectric oil-separating ring is provided at the internal surface of the core. The stator winding is made from conductors with an oval hollow shape provided with oil and paper insulation. The coil sides with their insulation are secured to the slots, made with rectangular cross section, by way of wedges. As coolant, oil is used both in the hollow conductors and in holes in the stator walls. Such cooling systems, however, entail a large number of connections for both oil and electricity at the coil ends. The thick insulation also entails an increased radius of curvature of the conductors, which in turn results in an increased size of the winding overhang.
The above-mentioned U.S. patent relates to the stator part of a synchronous machine which comprises a magnetic core of laminated sheet with trapezoidal slots for the stator winding. The slots are tapered since the need of insulation of the stator winding is smaller towards the interior of the rotor where that part of the winding which is located nearest the neutral point is disposed. In addition, the stator part comprises a dielectric oil-separating cylinder or ring nearest the inner surface of the core which may increase the magnetization requirement relative to a machine without this ring. The stator winding is made of oil-immersed cables with the same diameter for each coil layer. The layers are separated from each other by way of spacers in the slots and secured by wedges. What is special for the winding is that it comprises two so-called half-windings connected in series. One of the two half-windings is disposed, centered, inside an insulation sleeve. The conductors of the stator winding are cooled by surrounding oil. The disadvantage with such a large quantity of oil in the system is the risk of leakage and the considerable amount of cleaning work which may result from a fault condition. Those parts of the insulation sleeve which are located outside the slots have a cylindrical part and a conical termination reinforced with current-carrying layers, the purpose of which is to control the electric field strength in the region where the cable enters the end winding.
From CCCP 955369 it is clear, in another attempt to raise the rated voltage of the synchronous machine, that the oil-cooled stator winding comprises a conventional insulated conductor for medium voltage with the same dimension for all the layers. The conductor is placed in stator slots formed as circular, radially disposed openings corresponding to the cross-section area of the cable and with the necessary space for fixation and for coolant. The different radially disposed layers of the winding are surrounded by and fixed in insulated tubes. Insulating spacers fix the tubes in the stator slot. Because of the oil cooling, an internal dielectric ring is also needed here for sealing the coolant against the internal air gap. The design shown has no tapering of the insulation or of the stator slots. The design exhibits a very narrow radial waist between the different stator slots, which means a large slot leakage flux which significantly influences the magnetization requirement of the machine.
In a report from the Electric Power Research Institute, EPRI, EL-3391 from April 1984, an account is given of generator concepts for achieving higher voltage in an electric generator with the object being to connect such a generator to a power network without intermediate transformers. Such a solution is assessed in the report to offer good gains in efficiency and considerable financial advantages. The main reason that it was deemed possible in 1984 to start developing generators for direct connection to power networks was that a super-conducting rotor had been developed at that time. The considerable excitation capacity of the super-conducting field enables the use of an airgap-winding with sufficient thickness to withstand the electrical stresses.
By combining the concept deemed most promising according to the project, that of designing a magnetic circuit with a winding, known as a xe2x80x9cmonolith cylinder armaturexe2x80x9d, a concept in which two cylinders of conductors are enclosed in three cylinders of insulation and the whole structure is attached to an iron core without teeth, it was assessed that a rotating electric machine for high-voltage could be directly connected to a power network. The solution entailed the main insulation having to be made sufficiently thick to withstand network-to-network and network-to-ground potentials. Drawbacks with the proposed solution, besides its demand for a super-conducting rotor, are that it also requires extremely thick insulation, which increases the machine size. The coil ends must be insulated and cooled with oil or freons in order to control the large electric fields at the ends. The whole machine must be hermetically enclosed in order to prevent the liquid dielectric medium from absorbing moisture from the atmosphere.
The object of the present invention is to solve the above mentioned problems and to provide a rotating electric machine which permits direct connection to all types of high-voltage power networks.
This object is achieved by providing a conductor for high-voltage windings, as described herein with the advantageous features also described herein. A feature of the invention is that the electrical insulation between the strands of the conductor is made such that only certain strands are clad with an electrically insulating layer, and these strands which are provided with the insulating layer are so situated within the layers of the conductor that no two uninsulated strands come into electrical contact with each other.
Preferably, the electric conductor according to the invention has a number of twisted layers consisting of strands, of copper, aluminium or other suitable metal. However, contrary to conventional conductors for power transmission, the electrically conducting layer of the conductor is subjected to a magnetic field which induces currents, resulting in losses. In order to reduce these losses, therefore, the strands must be insulated from each other, but the requirement still remains that at least some strand in the outermost layer must be in contact with an inner semiconducting layer in the conductor""s insulation. Therefore, the conductor is further characterized in that at least one strand in the outermost strand layer is uninsulated.
The uninsulated strand or strands in the outer layer of the conductor defines the potential on the inner semiconducting layer and thereby the advantage is achieved that it ensures a uniform electric field within the insulation. By using uninsulated strands instead of insulated strands the advantage is also achieved of obtaining a less expensive insulated conductor for a winding.
As yet another advantageous feature the present invention, only a minority of said strands are uninsulated from each other.
As regards the rotating electric machine according to the present invention, it is characterized in that the winding has at least one current-carrying conductor that a first layer having semiconducting properties is provided around said conductor, that a solid insulating layer is provided around said first layer, and that a second layer having semiconducting properties is provided around the insulating layer.
Preferably, a rotating electric machine according to the present invention has a high-voltage insulated conductor/cable with a central electric conductor. Also the machine has a number of strands of copper (Cu), for instance, usually having a circular cross section. These strands are arranged in the middle of the high-voltage cable. Around the strands is a first semiconducting layer, and around the first semiconducting layer is an insulating layer, e.g. crosslinked polyethylene (XLPE) insulation. Around the insulating layer is a second semiconducting layer. Thus, in the present case, the concept of a xe2x80x9chigh-voltage cablexe2x80x9d normally used to denote a winding in a rotating electric machine does not include the outer protective sheath that normally surrounds such cables for power distribution. Furthermore, in a high-voltage cable for power distribution there is also an outer insulating layer on top of the second semiconducting layer, which is not included here.
By using conductors, of substantially the same type as conductors for transferring electric power, in the stator winding of the generator, in accordance with the invention, the advantage is achieved that the voltage of the machine is increased to such a level that it can be connected directly to the power network without intermediate transformers.
A rotating electric machine as described herein makes it is possible to have at least one winding system of conductors suitable for direct connection to distribution or transmission networks.
This also entails the further important advantage that the xcex94/Y-connected step-up transformer mentioned above can be omitted. Consequently, the solution according to the present invention represents major savings both in economic terms and regarding space requirement and weight for generator plants and other installations that use rotating electric machines.
To be able to cope with the problems which arise in case of direct connection of rotating electric machines to all types of high-voltage power networks, a machine according to the present invention may have a number of features which significantly distinguishes it from the state of the art both as regards to conventional mechanical engineering and the mechanical engineering which has been published during the last few years, some of which will follow below.
As mentioned, the winding is manufactured from one or more insulated conductors with an inner and an outer semiconducting layer, preferably an extruded cable of some sort. Some typical examples of such conductors are a cable of crosslinked polyethylene (XLPE) or a cable with ethylene propylene (EP) rubber insulation, which, however, for this purpose and according to the invention, has an improved design both as regards the strands of the conductor and as regards the outer layer.
The conductor is provided with an outer semiconducting layer with the aid of which its potential in relation to the surroundings shall be defined. This layer must therefore be connected to ground, at least somewhere in the machine, possibly only in the coil-end section.
The use of an insulated conductor with an outer semiconducting layer has the advantage that it permits the outer layer of the winding, in its full length, to be maintained at ground potential. Consequently, the invention may have the feature that the outer semiconducting layer is connected to ground potential. As an alternative, the outer layer may be cut off, at suitable locations along the length of the conductor, and each cut-off part length may be directly connected to ground potential.
A considerable advantage with having the outer layer connected to ground potential is that the electric field will be near zero in the coil-end region outside the outer semiconductor and that the electric field need not be controlled. This means that no field concentrations can be obtained, neither within the sheet, nor in the coil-end region, nor in the transition therebetween.
As another advantageous feature, at least two, and preferably all three, of the layers have substantially equal thermal expansion coefficients. Through this it is achieved that thermal movement is prevented and the occurrence of cracks, fissures or other defects in the winding due to thermal movement is avoided.
According to another characterizing feature each of the three layers is solidly connected to the adjacent layer along substantially the whole connecting surface. This has the advantage that the layers are fixed and unable to move in relation to each other and serves to ensure that no play occurs between the layers. It is very important that no air is allowed to enter in-between the layers since that would lead to disturbances in the electric field.
As mentioned, the present invention is intended for use at high voltages, which here refers primarily to voltages in excess of 10 kV. A typical operating range for a device according to the invention may be voltages from 36 kV up to 800 kV, preferably in the range 72,5-800 kV. In addition, the invention has the advantage to ensure uniform current distribution, counteract eddy-current losses and ensure that the inner semiconducting layer of the insulation system has the same potential during operation as the wire or strand layers in the conductor.
The material requirements for the insulating layer with regard to the XLPE process, for instance, are that it shall be stable, will not melt and is resistant to deformation at temperatures up to 220xc2x0 C. for approximately 30 min. Examples are enamelled wire, powder sintering (epoxy, high-temperature plastic), extruded high-temperature plastic, and oxide layers.
The above-mentioned object of the present invention is optimized by the various embodiments of the construction of the conductor as discussed herein.